The long-term goal of the project is to determine the structures and molecular mechanisms of catalysis for enzymes in the folate biosynthetic pathway, a proven target pathway for developing antimicrobial agents. The proposed research is to continue our current study on 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK), and to expand our research to include dihydroneopterin aldolase (DHNA). HPPK is an excellent model for studying mechanism of enzymatic pyrophosphoryl transfer. DHNA is a unique aldolase because it requires neither the formation of a Schiff base between the enzyme and its substrate nor metal ions for catalysis, and the enzyme also catalyzes the epimerization of its substrate. The central hypothesis behind the proposed research on HPPK, which is based on the results obtained in the previous funding period, is that HPPK undergoes dramatic conformational changes during its catalytic cycle and the conformational changes play critical roles in its catalysis. Thus, in Specific Aim 1, we will continue our quest for structure determination of HPPK along the catalytic cycle by X-ray crystallography. In Specific Aim 2, we will determine the conformational dynamics of the catalytic loops of HPPK by time-resolved fluorescence energy transfer (FRET) at equilibrium conditions and even as the reaction progresses and the dynamics of its core structure by heteronuclear NMR relaxation at the sub-nanosecond to nanosecond and microsecond to millisecond time scales. Most importantly, in Specific Aim 3, we will correlate the structure and conformational dynamics of HPPK with its catalysis by site-directed mutagenesis, biochemical analysis (particularly transient kinetic analysis), and biophysical methods. The main hypotheses behind the proposed research on DHNA are that (1) the two adlolases from Staphylococcus aureus and E. coil have different binding/catalytic properties and distinct responses to inhibitors and (2) general acid/base catalysis plays a most critical role in the catalytic mechanism of this unique aldolase. Thus, in Specific Aim 4, we will determine the structures of DHNA by X-ray crystallography, particularly the structures of the complexes with neopterin and monopterin, the closest mimics of the Michaelis complexes of DHNA. In Specific Aim 5, we will identify residues involved in general acid/base catalysis in DHNA by a combination of site-directed mutagenesis, transient kinetic pH-rate profile analysis, and NMR spectroscopic titration.